Chapter 1. Introduction to fakeroot

Fakeroot [1] is a utility that runs programs in an environment that looks as if they were
run with super-user privileges. It is used primarily for setting file ownerships and modes before packaging them. You
can for example create device nodes and store them in a tarball while logged in as a normal user. Of course, the
programs run from a fakeroot session cannot really do privileged system calls; fakeroot keeps an in-memory database of
file ownerships and such things.

Fakeroot was developed by the Debian Project [2] to help in building Debian packages. The
Debian packaging system needs a root environment so that it would be as easy as possible to set up ownerships and
permissions.

Fakeroot is released under the GNU General Public License version 2 [3].

Chapter 2. Fakeroot in Scratchbox

Scratchbox [4] introduces new requirements for fakeroot. During the development of Scratchbox
an enhanced version of fakeroot was developed with the name fakeroot-net. It was later merged with the upstream project and nowadays Scratchbox uses the
upstream codebase.

Differences between the default and the Scratchbox version are:

When using sbrsh [5] to implement CPU-transparency in Scratchbox, the command execution can
jump from host to target within a fakeroot session. Since both ends use the same filesystems (via NFS), they must also
use the same fakeroot session. This is not possible with the original design that uses SYSV IPC. The Scratchbox version
uses TCP/IP sockets for its internal communication. (The TCP version of the fakeroot command is also available in the
Debian package with the name fakeroot-tcp.)

Using TCP sockets in fakeroot is not enough to implement network-transparent fakeroot sessions. The sbrsh server
(sbrshd) is used to filter the information passed between the remote fakeroot environment and the fakeroot daemon
(faked) that keeps the database. The reason for this is explained in Chapter 5.

Fakeroot supports saving and loading its internal database in a file. The default file format uses inode numbers to
identify files. Scratchbox uses full path names instead of inodes so that it can reliably check the existence of the
files when loading a database. (The Debian binary package does not ship this version but the functionality is included
in the source package.)

Scratchbox provides the fakeroot command, the fakeroot daemon (faked) and a host version of the fakeroot library
(libfakeroot). They are sufficient for running host tools in fakeroot, but a target version of libfakeroot needs to be
installed for each Scratchbox target in order to run target binaries in fakeroot. Installing
Scratchbox [6] describes how to do that. Scratchbox's fakeroot is compatible with the
libfakeroot provided by the Debian package, so you can use that aswell.

As described above, the fakeroot daemon provided by the Debian package does not use the same database format as
Scratchbox's version. This should not be a problem though, since thanks to Scratchbox's binary redirection feature the
host version of the fakeroot command is normally used even when the target version is installed.

The fakeroot environment is imposed upon a process by using the C-library's LD_PRELOAD environment variable.
libfakeroot is preloaded by the dynamic linker whenever it loads a binary. This means that fakeroot does not work with
statically linked binaries.

There is also another side-effect. Since libfakeroot is loaded into the same process image with the "victim"
program, they share the same file descriptor table. Some programs (such as the configure scripts) use hard-coded
descriptor numbers. libfakeroot needs one file descriptor for its communication socket, and if the program starts to
use the same file descriptor, there will be trouble. fakeroot tries to monitor the status of its descriptor so that it
can open a new socket if the descriptor has been changed. If you start seeing messages about hijacked file descriptors,
you can try to make fakeroot use some other file descriptor with the --fd-base option. Its
default value is (descriptor_table_size - 100).

Chapter 3. Debugging

The fakeroot daemon can be launched with debug enabled and left running on the foreground:

$ faked --debug --foreground
33366:5027

The first number is the TCP/IP port it listens to, and the second number is its process ID. Now, in another terminal,
setup a fakeroot session manually that uses the daemon we started:

Now you can run programs in the hand-made fakeroot session and see the daemon's cryptic debug output in the other
terminal. This way you can also use a debugger to debug a program within a fakeroot environment.

/scratchbox/tools/lib/libfakeroot-tcp.so.0 is the host version. If you are running target
binaries, you should set LD_PRELOAD to /usr/lib/libfakeroot/libfakeroot-tcp.so.0.

When using a remote fakeroot session, the communication can be traced using the sbrsh daemon's debug log. See
Scratchbox Remote Shell [5] for instructions.

Chapter 4. Building fakeroot

This chapter contains instructions for building fakeroot from source code using Scratchbox's configuration options.
You shouldn't normally need to do that, since fakeroot is included in Scratchbox and all toolchains ship libfakeroot
binaries for their target architectures. See Installing Scratchbox [6] and Scratchbox toolchains [7] for more
information.

Fakeroot should be cross-compiled inside Scratchbox. The fakeroot source package is available in the /scratchbox/packages directory in the Scratchbox installation, but you can also download it from Debian
[7].

It is important to note that the Debian package uses the non-TCP version as the default fakeroot command. Also,
neither version is configured with the --with-dbformat=path option. You can change the
configure options by editing the debian/rules file. If you do that, you should also change
the package name and/or the package version to reflect the incompatibility with the standard Debian package.

The Debian package needs the "sharutils" package for running its tests. Scratchbox does not provide this package, so
you might first need to install it on the target:

Chapter 5. Implementation of network fakeroot

faked maintains a list of entries based on their device and inode numbers of the files that have been modified
during a fakeroot session. The entries contain a data structure that is essentially the same as the one used by the
stat system call. The TCP version introduces an additional remote field in the entry, which works like a "namespace" for the devices and inodes. All files on
the local filesystems belog to the default namespace (remote is not set).

When a remote command is run within a fakeroot session, sbrsh resolves the device numbers of the NFS filesystems
that are listed in its config file for the used target. If they are not exported by the local host but some third host,
it tries to find out if the NFS filesystems are mounted on the local host and use the device numbers of the mount
points.

sbrshd receives the list of mount entries and finds out what their device numbers are on the target device. Then it
creates a relay process that listens for connections from local
fakeroot sessions. When it receives one, it makes a corresponding connection to the faked running on the Scratchbox
host. It maintains as many connection pairs as there are processes running within the local fakeroot session. The relay
copies messages from the local session to the remote daemon and responses from the daemon to the session, and
translates the device numbers in the messages between the local and remote device number "spaces".

If the relay finds an unlisted device number in one of the incoming messages, it does not translate it but sets the
value of the remote field to the IP address of the host it is running at. This way faked
can serve unknown filesystems without the danger of device number/inode collisions.

Appendix A. fakeroot 1.2.3 manual page

fakeroot(1) Debian manual fakeroot(1)
NAME
fakeroot - run a command in an environment faking root privileges for
file manipulation
SYNOPSIS
fakeroot [-l|--lib library] [--faked faked-binary] [-i load-file] [-s
save-file] [-u|--unknown-is-real ] [-b|--fd-base ] [-h|--help ]
[-v|--version ] [--] [command]
DESCRIPTION
fakeroot runs a command in an environment wherein it appears to have
root privileges for file manipulation. This is useful for allowing
users to create archives (tar, ar, .deb etc.) with files in them with
root permissions/ownership. Without fakeroot one would need to have
root privileges to create the constituent files of the archives with
the correct permissions and ownership, and then pack them up, or one
would have to construct the archives directly, without using the
archiver.
fakeroot works by replacing the file manipulation library functions
(chmod(2), stat(2) etc.) by ones that simulate the effect the real
library functions would have had, had the user really been root. These
wrapper functions are in a shared library /usr/lib/libfakeroot.so*
which is loaded through the LD_PRELOAD mechanism of the dynamic loader.
(See ld.so(8))
If you intend to build packages with fakeroot, please try building the
fakeroot package first: the "debian/rules build" stage has a few tests
(testing mostly for bugs in old fakeroot versions). If those tests fail
(for example because you have certain libc5 programs on your system),
other packages you build with fakeroot will quite likely fail too, but
possibly in much more subtle ways.
Also, note that it's best not to do the building of the binaries them-
selves under fakeroot. Especially configure and friends don't like it
when the system suddenly behaves differently from what they expect.
(or, they randomly unset some environment variables, some of which
fakeroot needs).
OPTIONS
-l library, --lib library
Specify an alternative wrapper library.
--faked binary
Specify an alternative binary to use as faked.
[--] command
Any command you want to be ran as fakeroot. Use '--' if in the
command you have other options that may confuse fakeroot's
option parsing.
-s save-file
Save the fakeroot environment to save-file on exit. This file
can be used to restore the environment later using -i. However,
this file will leak and fakeroot will behave in odd ways unless
you leave the files touched inside the fakeroot alone when out-
side the environment. Still, this can be useful. For example, it
can be used with rsync(1) to back up and restore whole directory
trees complete with user, group and device information without
needing to be root. See /usr/share/doc/fakeroot/README.saving
for more details.
-i load-file
Load a fakeroot environment previously saved using -s from load-
file. Note that this does not implicitly save the file, use -s
as well for that behaviour. Using the same file for both -i and
-s in a single fakeroot invocation is safe.
-u, --unknown-is-real
Use the real ownership of files previously unknown to fakeroot
instead of pretending they are owned by root:root.
-b fd Specify fd base (TCP mode only). fd is the minimum file descrip-
tor number to use for TCP connections; this may be important to
avoid conflicts with the file descriptors used by the programs
being run under fakeroot.
-h Display help.
-v Display version.
EXAMPLES
Here is an example session with fakeroot. Notice that inside the fake
root environment file manipulation that requires root privileges suc-
ceeds, but is not really happening.
$ whoami
joost
$ fakeroot /bin/bash
# whoami
root
# mknod hda3 b 3 1
# ls -ld hda3
brw-r--r-- 1 root root 3, 1 Jul 2 22:58 hda3
# chown joost:root hda3
# ls -ld hda3
brw-r--r-- 1 joost root 3, 1 Jul 2 22:58 hda3
# ls -ld /
drwxr-xr-x 20 root root 1024 Jun 17 21:50 /
# chown joost:users /
# chmod a+w /
# ls -ld /
drwxrwxrwx 20 joost users 1024 Jun 17 21:50 /
# exit
$ ls -ld /
drwxr-xr-x 20 root root 1024 Jun 17 21:50 //
$ ls -ld hda3
-rw-r--r-- 1 joost users 0 Jul 2 22:58 hda3
Only the effects that user joost could do anyway happen for real.
fakeroot was specifically written to enable users to create Debian
GNU/Linux packages (in the deb(5) format) without giving them root
privileges. This can be done by commands like dpkg-buildpackage
-rfakeroot or debuild -rfakeroot (actually, -rfakeroot is default in
debuild nowadays, so you don't need that argument).
SECURITY ASPECTS
fakeroot is a regular, non-setuid program. It does not enhance a user's
privileges, or decrease the system's security.
FILES
/usr/lib/libfakeroot/libfakeroot.so* The shared library containing the
wrapper functions.
ENVIRONMENT
FAKEROOTKEY
The key used to communicate with the fakeroot daemon. Any pro-
gram started with the right LD_PRELOAD and a FAKEROOTKEY of a
running daemon will automatically connect to that daemon, and
have the same "fake" view of the file system's permissions/own-
erships. (assuming the daemon and connecting program were
started by the same user).
LIMITATIONS
Library versions
Every command executed within fakeroot needs to be linked to the
same version of the C library as fakeroot itself.
open()/create()
fakeroot doesn't wrap open(), create(), etc. So, if user joost
does either
touch foo
fakeroot
ls -al foo
or the other way around,
fakeroot
touch foo
ls -al foo
fakeroot has no way of knowing that in the first case, the owner
of foo really should be joost while the second case it should
have been root. For the Debian packaging, defaulting to giving
all "unknown" files uid=gid=0, is always OK. The real way around
this is to wrap open() and create(), but that creates other
problems, as demonstrated by the libtricks package. This package
wrapped many more functions, and tried to do a lot more than
fakeroot . It turned out that a minor upgrade of libc (from one
where the stat() function didn't use open() to one with a stat()
function that did (in some cases) use open()), would cause unex-
plainable segfaults (that is, the libc6 stat() called the
wrapped open(), which would then call the libc6 stat(), etc).
Fixing them wasn't all that easy, but once fixed, it was just a
matter of time before another function started to use open(),
never mind trying to port it to a different operating system.
Thus I decided to keep the number of functions wrapped by fake-
root as small as possible, to limit the likelihood of 'colli-
sions'.
GNU configure (and other such programs)
fakeroot, in effect, is changing the way the system behaves.
Programs that probe the system like GNU configure may get con-
fused by this (or if they don't, they may stress fakeroot so
much that fakeroot itself becomes confused). So, it's advisable
not to run "configure" from within fakeroot. As configure should
be called in the "debian/rules build" target, running
"dpkg-buildpackage -rfakeroot" correctly takes care of this.
BUGS
It doesn't wrap open(). This isn't bad by itself, but if a program does
open("file", O_WRONLY, 000), writes to file "file", closes it, and then
again tries to open to read the file, then that open fails, as the mode
of the file will be 000. The bug is that if root does the same, open()
will succeed, as the file permissions aren't checked at all for root. I
choose not to wrap open(), as open() is used by many other functions in
libc (also those that are already wrapped), thus creating loops (or
possible future loops, when the implementation of various libc func-
tions slightly change).
COPYING
fakeroot is distributed under the GNU General Public License. (GPL 2.0
or greater).
AUTHORS
joost witteveen
<joostje@debian.org>
Clint Adams
<schizo@debian.org>
Timo Savola
MANUAL PAGE
mostly by J.H.M. Dassen <jdassen@debian.org> Rather a lot mods/addi-
tions by joost and Clint.
SEE ALSO
faked(1) dpkg-buildpackage(1), debuild(1) /usr/share/doc/fakeroot/DEBUG
Debian Project 6 August 2004 fakeroot(1)